Production of small DNA phages with recombinant regulatory regions. We plan to produce phages with recombinant regulatory regions by genetic crosses between appropriate mutants of the closely-related phages S13 and phi-x174. It is now known that promoters lie in front of genes A, B and D. Also, the sequence data from Sanger's group indicates an unusual intercistronic region between genes F and G. Crosses will be carried out designed to yield phages which are recombinant in the A and the D promoter regions and in the region between genes F and G. Regulatory-region recombinants will be detected by slab gel analysis as a change in amount of synthesis of a specific protein. If a phage shows decreased expression of the expected gene the next step will be to determine whether the defect is in mRNA synthesis or only in protein synthesis. The aim is to sequence the DNA of an altered regulatory region by the Gilbert-Maxam method. Comparison with the corresponding sequences in the parental phages should show which regions of a promoter site or a ribosome binding site are essential to the function of that site.